Radio frequency choke coils



Sept. 24, 1957 G, ACKERLY ETAL 2,807,788

RADIO FREQUENCY CHOKE COILS Filed July 2, 1954 zal J4. ff y l 4. IL; l jIZ 1:: '1,7 l E: :55 l l 73 Y I7/ V//////,f: i i

UnitedStates Patent Y RADIO FREQUENCY CHoKE ColLs Applicatoin July 2,1954, Serial No. 441,112 s claims. (Cl. 336-69) This invention relatesto radio frequency choke coils and more particularly to wide-band radiofrequency choke coils for use in radio transmitters and other like radioequipment.

A radio frequency (R.F.) choke coil is used in a radio transmitter orlike equipment to feed the operating D.C. potentials to the electrodesof a vacuum tube. These coils offer a hiO-h impedance to the radiofrequency signal that is used, but a low impedance to the D.C. currentof the tube. Ordinarily in a transmitter these coils are used in theplate or anode circuit, the screen grid circuit-if a tetrode tube isbeing used-and in the input grid circuit. These choke coils should havea relatively high impedance with Yrespect to the radio frequency beingused to prevent undue losses of power in the circuit. When these chokecoils are used with a transmitter that is tunable over a relativelybroad band of frequencies the choke coils must present a relatively highimpedance to all of the different radio frequencies that are used.Because of series reso nant effects of R.F. choke coils at somefrequencies the choke coils may present an impedance that is much toolow, and adjustment of the coil must be made to prevent destruction ofthe coil itself or, at least, to prevent inei cient operation of theequipment.

Accordingly, it is one of the objects of the present invention toprovide a wide band radio frequency choke coil for use with wide-bandradio transmitters, eliminating large decreases in the impedance of thechoke coil at certain frequencies.

It is another object of the present invention to provide awide-bandradio frequency choke coil compensated for the decrease inimpedance caused by series resonant effects at certain frequencies.

It is yet another object of the present invention to provide a wide bandradio frequency choke coil wherein virnpedance variations with respectto frequency are minimized.

In accordance with the present invention there is provided a radiofrequency choke coil winding mounted upon a core of ferrite material, inoperation at higher R.F. frequencies an air-core may be used. Aconductive compensating member, usually in the form of a strip or wire,is connected to the high end of the winding (that is, the end of thewinding nearest in the particular kcircuit used to the electrodes of thetube being used). The compensating member is extended substantiallyparallel to the axis of and adjacent to the winding for a portion of thelength of the winding from the high end. It is believed that theconducting material placed along the winding acts somewhat as are-entrant transmission line to prevent harmonic standing waves frombuilding up along the winding and giving rise to the series resonant orseries tuning effectrthat makes the choke coil appear as a relativelylow impedance at some frequencies.

However, the invention both as to the organization and operation will bebest understood from the following description'when read in connectionwith the accompanying drawings; 'in which:

Figure l is aside view partially in section and partially broken away ofa R.F. choke coil embodying the invention; and

Figure 2 is a sectional view of the R.F. choke coil shown in Figurev 1taken through line 2 2 of Figure 1; and

Figure 3 is a side view partially in section and partially broken awayof a R.F. choke coil showing a further embodiment of the presentinvention; and

Figure 4 is a schematic diagram of an output power amplifier circuit ofa radio frequency transmitter showing the use of R.F. choke coilsembodying the present invention.

Referring now to the drawings wherein like elements are designated bylike reference characters throughout the figures and referringparticularly to Figures l and 2, a R.F. choke coil embodying the presentinvention includes an elongated cylindrical ferrite core 10 upon whichis wound in any suitable manner an inductance winding 12, here shownhelically wound.` To minimize losses and to increase the inductance ofthe choke coil the windings 12 are wound directly upon the ferrite core10 without an additional form. This is possible because although ferritehas a high magnetic permeability its electrical resistivity is alsogreat and it is an insulator to electric current. A low-loss plastictape 14 surrounds the winding 12, and the entire assembly is enclosed bya tubular cover 16 and end caps 18 and 20. The cylindrical cover 16 ismade of a low-loss plastic material, such as polystyrene; and the endcaps can be made of any non-magnetic material, such as Bakelite. The endcaps 18 and 20 each have a first recessed portion 22 and 24,respectively, into which the ends of the ferrite core rest; and a secondrecessed portion 26 and 28, respectively, into which the tubular cover16 tits. The cover 16 is fastened to the end caps 18 and 20 by anysuitable means, such as by cementing them together with a low-losscasting resin.

The end leads 30 and 32 of the winding 12 are brought out of the coverthrough apertures 34 and 36, respectively, and end lead 30 is fastenedto an upper terminal bolt 38 while end lead 32 is fastened to a lowerterminal bolt 40. The terminal bolts 38 and 40 also serve as connectingterminals to connect the choke coil into its operating circuit.

Terminal bolt 38 also is electrically and mechanically fastened to aconductive compensating member, in this case copper strip 42. The strip42 is positioned in a plane substantially parallel to the coil axisseparated from the coil by meansof a plastic tape strip 43 and extendsfrom the upper end of the winding parallel to the axis of the winding 12for a portion of the length of the winding. The strip 42 is held inplace along the winding 12 by plastic tape 14 wrapped around the winding12 and enclos ing the strip 42 and holding it in place. The `strip 42 isthe length of conducting material that provides compensation for theseries tuning effects of the coil that reduces itsirnpedance at certainfrequencies.

In operation the R.F. choke coil is placed in an operating circuit, aswill be more fully explained in Figure 3, with the end of the coil towhich the conducting strip 42 is connected nearest the electrode of thevacuum tube to which the voltage is being fed. Any radio frequency wavespresent at this high end of the choke coil will appear across the highimpedance of the coil and prevent losses of signal voltage by preventingthe signal voltage from appearing in other parts of the circuit, such asthe various A.C. and D.C. power supplies for the circuit. Although theoperation of the coil is not entirely understood it is believed that thestrip 42 operates somewhat as a re-entrant transmission line to preventharmonic standing waves from building up along the winding, whichstanding waves are believed to give rise to the low impedance seriestuning phenomena.

As an example, a R.F. choke coil was constructed having a winding 4%inch long of number 22 wire mounted on a 5/8 inch diameter ferrite rod.The winding was covered or wrapped with Teflon tape manufactured by E.l. Du Pont de Nemours and C'o., Wilmington, Delaware. A copper strip .02inch thick and 1/s inch Wide and 1% inches long was laid parallel to theupper end of the winding as shown, and held in place by Teon tape, Thecylindrical cover was made of polystyrene tubing, and the end caps ofBakelite. The conductance of the choke coil was measured at frequenciesfrom 2 to 30 megaeycles; and the reciprocal of the conductancewhich isthe measure of the effectiveness of the choke coil in ohms, as is wellknown-varied from 2,000,000 ohms at the lower frequency end of the bandand to 170,000 ohms on the high frequency end of the band. This istherefore a great improvement over certain known devices of this typewhose conductance varies to a greater extent, and at some points in the2 to 30 megaeycle band their conductance reciprocal drops to a very lowvalue causing poor performance of the circuit or possibly destructiveheating of the choke coil.

Referring now to Figure 3, a R.F. choke coil in accordance with thepresent invention again includes a ferrite core 50 upon which aninductanee Winding 52 is disposed. The winding 52 is again covered by alow-loss plastic tape 54 and the conductive compensating member is awire 56 adjacent to the plastic tape 54 and parallel to the axis of thewinding. The wire 56 is separated from the coil by a strip of plastictape 58 and low-loss tape 54 is wrapped around the winding S2 andenclosing the wire 56 in the same manner as shown in Figure l. The endcaps 62 and 64 fitted over the ends of the ferrite core 50 are notneeded after the choke coil is completed but are used in constructingthe coils and are left on as an economy measure. The lower end lead 66is taken out through aperture 68 in end cap 64, and the Wire 56 is takenout through aperture 70 in the end cap 62. The upper end lead 72 isfastened to the wire 56 by any suitable means such as soldering.

To provide dampening of high frequency reflections across the choke coilthe lower end of the winding 52 is coated for a short distance of itslength with aquadag 73 (a graphite coating material) which causes highfrequency losses and substantially damps out any high frequencyreflections. The aquadag 73 is placed on the winding 52, of course,before the plastic tape 54 is wrapped around winding 52. The entirestructure with the exception of end lead 66 and the upper portion ofwire 56 is then embedded in a low-loss casting resin, such as Aralditemade by the Ciba Co., Inc., of New York, New York, and the casting resinafter it hardens forms the case 80. The end lead 66 and the wire 56 thenform the connections to the coil.

The R.F. choke coil operates in the same manner as the coil shown inFigures l and 2, and the wire 56 is connected nearest the electrode ofthe particular tube being used in the circuit as will be more fullyexplained hereinafter. Again as an example a R.F. choke coil wasconstructed in accordance with the embodiment shown in Figure 3 having aferrite rod Mi inch in diameter as a core. A winding of number 32 wire,27A@ long was placed on the core. Aquadag was painted around the one endof the Winding for 1A inch of its length. The winding was wrapped withBi-Seal electrical tape, made by the Bishop Gutta Percha Co., CedarGrove, New Jersey, and a length of number wire was laid parallel to thewinding for 15/16 inch along the winding from the en'd opposite to theaquadag coating and taped in place with the Bi-Seal electrical tape. Theentire choke coil Was then molded in Araldite casting resin. In the bandfrom 2 to megacycles the reciprocal of the conductance of this chokecoil varied from 1,400,000 ohms to 330,000 ohms, again much better thancommercially available R.F. choke coils.

Referring now to Figure 4 a `schematic diagram of a well known type ofpower amplifier for a wide-band transmitter is shown. The operation ofthis circuit is well known, but a brief explanation of the functions ofthe various components will be given. The input signal to the amplifier(which may be from a buffer amplifier or an oscillator, as is Wellknown), is introduced across terminals and 92, terminal 92 being groundand terminal 90 being connected with the grid 94 of an amplifier tube96. The signal is coupled through a capacitor 98 and through aninductance-resistanee network 100 to the grid 94. Theinductanee-resistance network 100 is a network used to suppressparasitic oscillations. The signal is amplified through the tube 96 andappears across an output circuit made up of a parallel connected tuningcapacitor 102 and an output inductor 104. The signal is then coupled bya coupling coil 106 to any suitable antenna (not shown) to propagate thesignal. Connected to the anode 108 of the tube 96 is a secondinductanceresistanee network 110 which is also used to suppressparasitic oscillations. The grid 94 is biased by a suitable D.C. biassupply (not shown) through a resistor 112 and a choke coil 114 embodyingthe present invention. Capacitor 116 is a by-pass capacitor for radiofrequency signals. The filament cathode 118 of the tube 96 is heatedthrough a transformer 120 connected to a suitable source of A.-C. heatersupply voltage (not shown). A D.C. ground return for the operatingcurrent of the tube 96'is provided by connecting a lead 122 from acenter tap 124 on the secondary of the transformer 120 to ground.Condensers 125 and 126 connected to either side of the filament 118 arebypass Condensers to prevent any radio frequency current from appearingin the transformer 120 and to provide a signal ground for the filamentcathode 118. Operating voltage for the screen grid 128 of the tube 96 isprovided from a D.C. screen grid supply (not shown) through a resistor130 and a second choke coil 132 embodying the present invention.Condensers 134 and 136 connected between either side of the second R.F.choke coil 132 and ground are radio frequency bypass Condensers. TheD.C. operating potential for the anode 108 is supplied by a D.C. platesupply (not shown) through a third choke coil 138, embodying the presentinvention, through the output inductor 104, and through the plateparasitic oscillations suppressor inductor 110 to the anode 108. BypassCondensers 140 and 142 are connected from either end of the third R.F.choke coil to ground.

By use of the choke coils 114, 132 and 138 in the grid, screen and platecircuits respectively, embodying the invention, the transmitter may betuned over a relatively broad band of frequencies (2 to 30 megaeycles,for instance) without changing or adjusting the choke coils which hasheretofore not been practical.

Radio frequency choke coils for use with radio transmitters and likeequipment constructed in accordance with the invention are characterizedby physically small size because of the use of the ferrite core, and bya cornpensating system that allows the choke coil to be used over a wideband of frequencies without having its impedanee appreciably lowered bythe series resonant or series tuning phenomena.

What is claimed is:

l. A radio frequency choke coil comprising in combination, a highfrequency magnetically conductive core member, an inductanee winding onsaid core member, and an elongated conductive compensating member havingone end thereof electrically connected with one end of said winding andextending therefrom for a portion of the length of said winding adjacentto and in a plane substantially parallel with the axis of said winding.

2. A radio-frequency choke coil comprising in cornbination, a ferritecore member, an inductanee winding on said core member, low-losselectrical insulation means surrounding said winding, an elongatedconductive compensatng member having one end thereof electricallyconnected to one end of said winding extending from said one end of saidwinding for a portion of the length of said winding adjacent to and in aplane substantially parallel with the axis of said winding.

3. A radio frequency choke coil as deined in claim 2 wherein saidwinding has means providing a coating material extending for a smallportion of its length from the end of said Winding opposite to whichsaid compensating member is connected for damping high frequencyreflections along said winding.

4. A radio-frequency choke coil as defined in claim 2 wherein saidwinding has a graphite coating extending for a portion of its lengthfrom the end of said winding opposite to which said compensating memberis connected, and wherein said choke coil elements are embedded as aunit in a body of low-loss casting resin.

5. A radio-frequency choke coil comprising a helically tion of thelength of said inductance element.

References Cited in the file of this patent UNITED STATES PATENTS2,227,493 Finch Ian. 7, 1941 2,352,166 Camilli I une 27, 1944 2,381,782Stephens Aug. 7, 1945 2,498,561 Lipkin Feb. 21, 1950 2,522,731 WheelerSept. 19, 1950 2,701,335 Sargeant et al Feb. 1, 1955

